feat(subsonic)!: drop support for guessed artist covers in filesystem

it doesn't make sense with multi-artist support anymore

gonic will use lastfm or an album cover instead

Release-As: 0.16.0

server/ctrlsubsonic/testdata/test_search_three_q_art

@@ -6,9 +6,24 @@
     "serverVersion": "",
     "searchResult3": {
       "artist": [
-        { "id": "ar-1", "name": "artist-0", "albumCount": 3 },
-        { "id": "ar-2", "name": "artist-1", "albumCount": 3 },
-        { "id": "ar-3", "name": "artist-2", "albumCount": 3 }
+        {
+          "id": "ar-1",
+          "name": "artist-0",
+          "coverArt": "ar-1",
+          "albumCount": 3
+        },
+        {
+          "id": "ar-2",
+          "name": "artist-1",
+          "coverArt": "ar-2",
+          "albumCount": 3
+        },
+        {
+          "id": "ar-3",
+          "name": "artist-2",
+          "coverArt": "ar-3",
+          "albumCount": 3
+        }
       ]
     }
   }